The present invention relates to a circuit for protecting a load from an overvoltage which may possibly be applied from a power supply, and more particularly to an overvoltage protecting circuit for circuit elements with relatively low withstand voltage such as MOS transistors used as electronic parts for sensors, control equipments or the like to be mounted on vehicles.
Electronic circuits mounted on vehicles such as automobiles or motor vehicles are required to operate under the presence of relatively large variations in power supply voltage. Therefore, various overvoltage protecting circuits have hitherto been used. One of those circuits is a circuit, as shown in FIG. 10, which uses a Zener diode ZD1 and a current limiting resistor RA1.
The circuit shown in FIG. 10 is one kind of general constant voltage circuit in which by virtue of a voltage applied to a power supply receiving terminal VCC, a current is caused to flow in the Zener diode ZD1 through the current limiting resistor RA1. Even when an overvoltage is applied, the voltage of a load power supply terminal VCCS is clamped by the Zener voltage of the Zener diode ZD1, thereby providing a protection against the overvoltage.
In the circuit of FIG. 10, however, so long as the overvoltage is applied, a large current continues to flow through the resistor RA1 and the Zener diode ZD1 with remarkable heat generation. When the application of the overvoltage extends over a long time, there is a fear that the Zener diode results in breakdown.
In a first method for suppressing such fear of breakdown, the electric capacity of the Zener diode ZD1 and the resistor RA1 is made large so as to be sufficiently-proof against an overcurrent. There is a second method in which the resistance value of the current limiting resistor RA1 is made large.
However, the first method is not preferable since it brings about the increase in size and cost of parts. In the second method, on the other hand, a voltage drop caused by the current limiting resistor RA1 becomes large to narrowly restrict an operating voltage range in which a constant voltage characteristic based on the Zener diode ZD1 is obtained. Further, the large voltage drop caused by the current limiting resistor RA1 deteriorates a voltage variation or regulation characteristic even in a normal operating condition. Accordingly, the second method is not also preferable.
As one prior art, JP-A-9-307361 has proposed an overvoltage protecting circuit which uses an overvoltage detecting circuit composed of a resistor and a Zener diode and a switching circuit composed of bipolar transistors.
The overvoltage protecting circuit disclosed by JP-A-9-307361 is provided for the purpose of protection of a microwave EFT (Field Effect Transistor). When an overvoltage exceeding the sum of the Zener voltage of the Zener diode and the base-emitter voltage of the switching transistor is applied to a power supply receiving terminal, the switching circuit is operated so that a load is cut off from a power supply line to protect the load.
The prior art mentioned above has a problem that the IC configuration or implementation of the circuit is not taken into consideration and the application to a protecting circuit for automobile control IC is therefore difficult.
Namely, control IC""s and signal processing circuit IC""s for sensors presently used in automobiles are dominated by IC""s based on the integration of MOS transistors. However, in the overvoltage protecting circuit of the above-mentioned prior art, the bipolar transistors and the Zener diode are used as circuit elements. Therefore, it is difficult to apply the prior art to a protecting circuit for automobile control IC. In the following, this will be described in detail.
In general, a control IC is constituted by an integrated circuit formed using an MOS transistor manufacture process. Accordingly, in order that a protecting circuit for such a control IC is also accepted in the same chip to attain the reduction in size and cost, it is preferable that the protecting circuit can be realized in like manner by use of a standard MOS transistor manufacture process.
However, the standard MOS transistor manufacture process used for IC""s such as general microcomputers does not include a process step for forming a Zener diode and bipolar transistors. Therefore, the application of the above-mentioned prior art to a protecting circuit for automobile control IC requires the addition of another manufacture process. As a result, such application is difficult.
Also, when an overvoltage protecting circuit is to be realized by a CMOS process for microcomputers, it is necessary to take a special measure against a latch-up phenomenon peculiar to CMOS transistors. In the above-mentioned prior art, however, this is not taken into consideration.
Further, electronic parts such as microcomputers for control and sensors are generally applied with a power supply voltage of about 5 V. In the case where they are used in an automobile, it is necessary to use a regulation circuit so that a voltage regulated from a battery voltage (normally about 12 V) to about 5 V is supplied as the power supply voltage.
In this case, it is desired to prevent the electronic parts from being broken down even when the regulation circuit is broken down or when the battery voltage is directly applied erroneously at the time of checking/maintenance. In the above-mentioned prior art, however, this is not taken into consideration.
Furthermore, in the case where such electronic parts are used in an automobile, its actual working environment is considerably severe or it is required to sufficiently make the protection from breakdown caused by a surge voltage and electromagnetic noises generated at an ignition system of an engine and/or the prevention of erroneous operation caused thereby. In the above-mentioned prior art, however, this is not taken into consideration.
An object of the present invention is to provide an overvoltage protecting circuit which overcomes the above-mentioned problem of the prior art and is suitable for integration or IC circuit configuration.
To attain the above object, the present invention provides an overvoltage protecting circuit comprising an overvoltage detecting circuit for detecting the appearance of an overvoltage by comparing a voltage of a power supply receiving terminal with a reference voltage, and a switching element connected between the power supply receiving terminal and a load power supply terminal and on/off controlled in accordance with an output signal of the overvoltage detecting circuit, the reference voltage being provided by a source-drain voltage of an MOS transistor.
The overvoltage protecting circuit of the present invention can be provided with the following construction.
(1) A diffused resistor is connected in series with the MOS transistor.
(2) At least the overvoltage detecting circuit and the switching element are formed, together with a load connected to the load power supply terminal, on the same chip by an MOS transistor manufacture process.
(3) At least the switching element and transistors included in the overvoltage detecting circuit are constituted by PMOS transistors.
(4) The transistor may include a high withstand voltage MOS transistor for EPROM.
(5) A path extending from the power supply receiving terminal to the load power supply terminal through the switching element is provided in a form divided into at least two or more systems. One overvoltage detecting circuit may serve for the two or more systems.
(6) A shunt element is connected in parallel between the load power supply terminal and the ground.
(7) An internal or integral power supply is provided between the load power supply terminal and the ground. The internal power supply may include a condenser.
(8) At least the switching element includes the series connection of at least two MOS transistors.
(9) The load formed on the same chip may comprise a signal processing chip including at least an analog circuit, a logic circuit and a memory circuit.
(10) The load formed on the same chip may comprise a sensor chip including at least an analog circuit, a logic circuit, a memory circuit and a sensor element.